This invention relates to apparatus for investigating subsurface earth formations with electromagnetic energy, and, more particularly, to antenna apparatus for use in downhole logging tools to investigate properties of such formations.
An open hole logging technique has long been desired in the search for oil and gas which would assist in determination of total formation water volume or saturation. One purpose of this is to differentiate water and oil zones to determine whether a formation should be primarily water or hydrocarbon bearing. For example, by measuring formation porosity, which responds to total fluid content, hydrocarbon fluid volume may be approximated by subtracting the bulk water volume.
In the past bulk volume of water was measured by relying on the difference in resistivity between water and hydrocarbon bearing formations. Thus, technology known as resistivity logging was developed for measuring such formation resistivity differences by a variety of techniques. For example, induction logging sought to measure electrical conductivity of formations by inducing currents of relatively low frequency on the order of 20,000 Hz into the formation and measuring the return current.
A problem arises however when water salinity is rather low. This contrast between resistivity of water bearing and other formations was exhibited only with relatively saline water and was greatly diminished with fresh formation water. Accordingly, conventional resistivity logging instruments had difficulty in differentiating hydrocarbon and fresh-water bearing intervals, and other techniques were thus sought after.
One method which has met with some success, known as dielectric logging, seeks to measure the dielectric constant or permitivity of formations. Although at 20 KHz, the electromagnetic field in a borehole is relatively independent of the dielectric constant, at frequencies above 10 MHz the influence of dielectric properties of the formation on electromagnetic energy becomes significant. More importantly, however, this parameter has been shown to be substantially constant for water of varying degrees of salinity but substantially variable from that of other formation materials commonly encountered.
For example, at 47 MHz, the dielectric constant is approximately 81 for fresh water, and 78 for saline water (105 ppm), whereas the dielectric constants for oil, limestone, and shale, are approximately 2-4, 6-8, and 10-15, respectively.
From the foregoing, it can be appreciated that measurement of relative dielectric constants of formations by logging instruments might be employed to good effect to obtain reliable estimates of water saturation even for areas of brackish or low formation water salinity. The dielectric log was thus useful where resistivity logs are inadequate.
The technique of measuring electrical permittivity or the dielectric constant of a formation basically involved measuring the travel time (or alternatively the phase shift) and the attenuation of an electromagnetic field propogating through the formation with a known angular frequency. General discussion of the fundamental principles and apparatus conventionally employed in this type of logging may be found, for example, in U.S. Pat. No. 3,944,910 to Rau, and in a technical bulletin entitled "Dielectric Log", pages 1-9, copyright 1981 by Dresser Atlas, Dresser Industries, Inc., which are herein incorporated by reference.
The dielectric constant of a lossy material can be expressed as a complex quantity of the form .epsilon.*=.epsilon.'+j.epsilon.", wherein
.epsilon.' represents the "true" dielectric constant of the materials in lossless form (e.g., displacement currents for a particular electric field if lossless), and
.epsilon." represents the "loss factor" of the material (losses due to conduction and relaxation effects, e.g., dipolar relaxation losses).
However, it has long been known in the art that subsurface formation materials have appreciable conductivity. Therefore, oftentimes .epsilon." is significantly greater than .epsilon.'. Because .epsilon." is thus necessarily measured to some extent in subsurface formations when attempts are made to measure .epsilon.', accuracy of this measurement of .epsilon. has frequently been found difficult.
A "loss tangent" term has been known to contribute to this loss factor term .epsilon.". The term may be thought of as a ratio of lossy conduction current (.sigma.) to displacement current (.omega..epsilon.), e.g., a measurement of relative conduction losses, and may be found defined in the literature as: EQU Loss Tangent=.sigma./.omega..epsilon.
From the foregoing, it may be seen that when .sigma. is relatively small, this loss tangent may be for practical purposes neglected in a measurement of .epsilon.'. However, when .sigma. becomes significant as in the case of typical well logging conditions, this loss tangent may be kept low in order to permit measurement of .epsilon.' and to neglect the effects of .epsilon." by making the measurement frequency .omega. large. Thus, as .omega. goes above 500 MHz, .epsilon.' becomes increasingly greater than .epsilon." so that reliable measurements of .epsilon.' become possible. As this .omega. goes even higher into the GHz range, .epsilon.' becomes substantially greater than .epsilon.".
This would suggest using as an investigation frequency of the propagating electromagnetic wave the highest frequency possible. However, several problems have been encountered for extremely high frequencies of investigation. For example, in approaching the GHz range, the wavelength .lambda. of the propagating electromagnetic energy is very small and can approach the thickness of mudcake encountered in the borehole. In these circumstances, the mudcake has been found to act in the manner of a wave 9uide in propagating a portion of the transmitting energy. Accordingly, this decreases the amount of energy available to flow out into and through the formation of interest, known as the lateral wave. It will be recalled that it is the attenuation and phase difference encountered in this wave traversing a portion of the borehole of interest which is required to derive a reliable measurement of the dielectric constant of that material through which the lateral wave travels.
Notwithstanding the foregoing, attempts have nevertheless been made to develop a successful logging technique for operating at frequencies as high as 1.1 GHz as in the case of the aforementioned patent to Rau.
In the use of extremely high investigation frequencies, it is conventional to provide as transmitting and receiving antennas for such electromagnetic energy antennas known as resonant cavities which are loaded up with a dielectric material so as to resonant at the desired frequency. At a frequency of 1.1 GHz a half wave length is approximately 12 in. or 7.5 cm in length. In accordance with microwave theory, it is known that a cavity having dimensions approximating the half wave length may be made to resonant efficiently. Unfortunately, due to the physical constraints of logging tools, antennas having half wave length dimensions were obviously impracticable.
However, it has also long been known in the art that the required cavity dimensions for efficient radiation may be reduced by loading up the cavity with a dielectric material. In this manner the cavity dimension may be reduced by a factor of 1/.sqroot..epsilon., whereby a cavity dimension would thereby become .lambda./2.sqroot..epsilon.. In the example under consideration a 12 in. cavity may thus be reduced in size to a loaded cavity of more reasonable proportion of 3 in. by filling the cavity with a dielectric material having a dielectric constant of 16 (i.e., 3"=12"/.sqroot.16). In particular, with respect to an embodiment depicted in the '910 patent, a cavity was in fact provided fed by a probe in which the cavity was entirely filled with a material having a dielectric constant of 4. This provided a cavity of reasonable dimension resonating for maximum pick up and transmission at the desired frequency.
At lower frequencies such as a nominal frequency of 200 MHz, the half wave length is approximately 30 in. Still operating under the physical constraint of reasonable logging tool dimensions, in order to reduce the cavity size by a factor of ten from 30 in. to 3 in., based on the foregoing relation, this would require a loaded reasonant cavity wherein the dielectric material had a dielectric constant of 100 (i.e., .sqroot..epsilon.=10). However, in practice, it is extremely difficult to provide an appropriate material for filling the cavity having a dielectric constant of 50 or greater let alone one which provided a constant on the order of 100 or the like.
Yet another problem with cavity backed slot antenna design suitable for use in dielectric logging is the necessity for accommodating tremendous pressure differentials which may exist between the cavity and the borehole, such differentials at times equaling or exceeding 20,000 psi. With respect to the loaded resonant cavity approach for extremely high investigating frequencies wherein such loading is possible, this problem has not so substantial inasmuch as the cavity openings could be filled with a water-tight ceramic insulating material having the appropriate dielectric constant filling the cavity and the antenna slot. However, in the lower frequencies such sealing frequently became a serious problem.
The disadvantages of the prior art are overcome by the present invention and an improved antenna apparatus for use downhole in the well logging of subsurface earth formations is hereinafter disclosed, including a novel transmitter and receiver antenna assembly construction with improved pressure sealing characteristics.